This invention relates generally to mass transfer columns and, more particularly, to apparatuses and methods for collecting and mixing descending liquid for more uniform distribution to an underlying bed of packing or other devices within such columns.
Mass transfer columns, including heat exchange columns, typically include an upright shell and a plurality of zones within the shell where packing and/or horizontally disposed trays are used to facilitate mass or heat transfer between fluid streams flowing within the column. The fluid streams are normally one or more downwardly flowing liquid streams and one or more ascending vapor streams, although other combinations of fluid streams are possible. Liquid exiting the bottom of one zone may have different concentrations and compositions at different locations across the horizontal cross section of the zone. In order to reduce these concentrational and compositional maldistributions, the liquid is often collected and mixed before it is then distributed to an underlying zone. Separate components are frequently used to effect the desired collection, mixing and distribution of the liquid as it descends from one zone to another. The use of separate components, however, can be undesirable because the vertical spacing occupied by these components reduces the available area within the column for other processing of the fluid streams and may require that a taller column be utilized to provide the spacing needed to effect the desired processing operations.
A combined collector and mixer has been utilized in columns of the type described above in order to reduce the number of components and the vertical spacing required to collect and mix the liquid exiting from a zone within the column. This combined collector and mixer utilizes a plurality of rows of upwardly extending vanes that collect the descending liquid and feed it into sumps that in turn feed the liquid into a center downcomer. While this device is an effective mixer because all of the collected liquid flows to a single downcomer, mixing efficiency would be greatly reduced in applications where high liquid flow rates and/or large column diameters require the use of two or more downcomers. In such applications, liquid, which is fed from one area of the collector into one downcomer, can have a different composition than liquid flowing from the remaining area of the collector into the other downcomer. As a result, a need has developed for a combined collector and mixer that is not only capable of handling high liquid flow rates using two or more downcomers but is also effective in mixing liquid from different areas of the collector so that liquid entering the downcomers is of substantially uniform composition.